Optimal arrangement of milk vetch plantation based on land suitability and carbon sequestration in croplands:A case study of Pucheng County, Fujian Province

Soil carbon sequestration is an important way to reduce greenhouse gas emissions. Milk vetch and other green manures importantly impact annual variations of CO₂ concentration in the atmosphere by promoting soil organic carbon accumulation and improving cropland soil fertility. However, research on the optimal arrangement of milk vetch plantation had to-date been hardly integrated with the suitability croplands for milk vetch plantation and realistic need for soil carbon sequestration. This study aimed to arrange milk vetch in the most suitable areas and fix as much CO₂ as possible. Thus the suitability of milk vetch plantations in croplands was evaluated by analyzing soil organic carbon density (SOCD) in croplands based on the 1:50 000 scale database of land use/soil pattern in Pucheng County, Fujian Province. The optimal arrangement of milk vetch plantations in Pucheng was done using an integrative method in GIS environment. Then the modificatory weighted index sum method and dynamic clustering analysis were used for both land suitability and SOCD by dividing the index based on the principle of land suitability for milk vetch plantation and carbon sequestration needs of croplands. The priority, sub-priority and general plantation areas of milk vetch in Pucheng were optimally arranged based on the principles of preferential arrangement of croplands with low topsoil SOCD and high suitability for milk vetch growth. The results showed that 81.82% of the croplands in the study area had various suitability of milk vetch plantation. If milk vetch was planted in all the cultivated lands in the study area, the annual total yield of fresh milk vetch and application rate of green manure were 657 056.65 t and 27.75 t·hm^-2, respectively; the annual carbon sequestration and the cultivated land area for which milk vetch pro-vided organic carbon sources would be 29 900.74 t and 23 677.72 hm², respectively. The range of SOCD in the study area was 2.50-5.74 kg·m^-2, differing sharply in space. On the basis of total cropland in Pucheng County, optimal area arrangement for milk vetch plantation was 59.72%, of which priority and sub-priority regions dominantly accounted for 25.72% and 50.34%, respectively. Priority region was mainly distributed in the basin area, sub-priority region distributed in the 200-400 m altitude range with gentle slope terraces, while general region was located in 250-800 m altitude range on slope terraces. The towns (including Liantang, Shuibei, Gulou, Yongxing and Zhongxin), which most needed soil carbon sequestration, were planned as the key developing areas. Then towns (e.g., Fuling, Xianyang, Shipi and Jiumu) were planned as the reserve areas for milk vetch plantation. The study provided the guide for the efficient planting and utilization of milk vetch. This showed the need to increase scientific implementation of soil organic matter programs and the continuous improvement of farmland quality by preferentially selecting suitable areas for milk vetch planting based on land suitability and carbon sequestration needs of croplands.